I've read that C3s cannot synthesize foods under extreme conditions like high light intensity and temp. Why ? ? ? Is it due to loss of optimum conditions enzymes get denatured and high light intensity remove more electrons than reqired from some biomolecules leaving them useless ? ? ?
But, this is generalised cause, so C4s should also stop food production in such conditions, but they don't , So, Are their enzymes adapted for those conditions also ? ? ?
Why ? ? ? They are not desert plants, i mean . And what about light intensity ??? Are their molecules adapted for that also ? ? ?

The assimilation of carbon in C3 plants is much slower than that in C4 plants. The plant can not produce glucose fast enough so, at high temperatures, when NADPH is formed much faster, the process slows down.
I don't think i've explained it very well. SOrry if you don't understand

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

The closing of the stomas is a cell response to the lack of water, as in high temperatures a great deal of water is lost through transpiration. C3 plants aren't addapted to reduce transpiration. A side effect is excess O2

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

Probably now i'm nearer to the ans.Thanks for help.
But, see photosynth, gives O2 and respir. gives CO2 then how can i say with 100% surity that closure of stomata leads to excess O2ation. SORRY FOR LATE RESPONSE.

I'm not sure if this answer is what you are looking for.
rate of photosynthesis is greater than rate of respiration.
Think that. if the case was not like that, photosynthetic organisms wouldn't be able to produce O2 at normal conditions too.

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I'm affraid you missunderstand. Closing of a plant stomatas doesn't lead to excess O2 as in the fact that the rate of fotosynthesis is greater than that of respiration. Photosynthesis is dependent of CO2. If the stomatas are closed then there is no CO2 coming in from the outside. That is why, at very bright light when c3 plants close their stomatas the rate of photosynthesis drops

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter